The present invention relates to a pixel structure for a liquid crystal display device which employs pixels each composed of a plurality of subpixels and permits a multi-gradation display. More particularly, the invention concerns a pixel structure which provides for improved multi-gradation display quality and aperture ratio.
A prior art example of such a pixel structure is disclosed in U.S. Pat. No. 4,840,460. As depicted in FIG. 1A which is a perspective view showing one pixel region of a liquid crystal display panel cut out perpendicularly thereto, a control capacitor electrode 2 is formed on the interior surface of a transparent base plate 1 as of glass and an insulating film 3 is deposited over the entire area of the interior surface of the transparent base plate 1 including the control capacitor electrode 2. On the insulating film 3 there are formed four equally divided square subpixel electrodes 4.sub.1 to 4.sub.4. On the interior surface of a transparent base plate 5 as of glass, disposed in adjacent but spaced relation to the subpixel electrodes 4.sub.1 to 4.sub.4, there is deposited a common electrode 6 to define between it and the subpixel electrodes 4.sub.i (where i=1 to 4) a space in which liquid crystal 7 is sealed The control capacitor electrode 2, the subpixel electrodes 4.sub.i and the common electrode 6 are transparent electrodes formed of ITO or similar material. Thus, one pixel is divided into four subpixels F.sub.1 to F.sub.4 corresponding to the subpixel electrodes 4.sub.1 to 4.sub.4, respectively. As shown in FIG. 1B, a control capacitor C.sub.Ci using the insulating film 3 as a dielectric is formed between each subpixel electrode 4.sub.i and the control capacitor electrode 2, and a liquid crystal capacitor C.sub.LCi using the liquid crystal 7 as a dielectric is formed between the subpixel electrode 4.sub.i and the common electrode 6. FIG. 2 shows an electric equivalent circuit of the pixel depicted in FIG. 1A. Letting electrostatic capacitances of the control capacitor C.sub.Ci and the liquid crystal capacitor C.sub.LCi be represented by C.sub.Ci and C.sub.LCi for convenience sake, the partial area of the control capacitor electrode 2 overlapping each subpixel electrode 4.sub.i is so adjusted as to satisfy the following condition: EQU C.sub.C1 &gt;C.sub.C2 &gt;C.sub.C3 &gt;C.sub.C4 ( 1)
The control capacitor electrode 2 is connected as shown in FIG. 1B to a drain electrode D of a thin film transistor (hereinafter referred to as a TFT) 8 formed on the transparent base plate 1 adjacent the pixel in FIG. 1A. A predetermined voltage Va is applied across the control capacitor electrode 2 and the common electrode 6 via the TFT 8. When the TFT 8 is turned ON, the applied voltage Va is divided, for each subpixel F.sub.i, into a voltage V.sub.Ci across the control capacitor C.sub.Ci and a voltage V.sub.LCi across the liquid crystal capacitor C.sub.LCi. The voltage V.sub.LCi is expressed as follows: ##EQU1## By setting the capacitance of each control capacitor C.sub.Ci to such a value as in Eq. (1), the voltage V.sub.LCi across each liquid crystal capacitor C.sub.LCi can be set to satisfy the following condition: EQU V.sub.LC1 &gt;V.sub.LC2 &gt;V.sub.LC3 &gt;V.sub.LC4 ( 3)
Letting a voltage at which the transmission of light through the liquid crystal is saturated be represented by V.sub.U and a threshold voltage by V.sub.L, the Voltage V.sub.LCi across the liquid crystal capacitor C.sub.LCi can assume the following cases according to the value of the voltage Va applied to the pixel, as shown in FIG. 3.
(a) V.sub.LCi =0 for Va=0.
(b) V.sub.LCi =V.sub.U and V.sub.LC2=V.sub.L : In this case V.sub.LC3 and V.sub.LC4 are lower than V.sub.L. The applied voltage Va in this intance is indicated by Va1.
(c) V.sub.LC2 =V.sub.U and V.sub.LC3 =V.sub.L : The applied voltage Va in this instance is indicated by Va2.
(d) V.sub.LC3 =V.sub.U and V.sub.LC4 =V.sub.L : The applied voltage in this instance is indicated by Va3.
(e) V.sub.LC4 =V.sub.U : The applied voltage Va in this instance is indicated by Va4.
The applied voltage Vai is as follows: EQU Va1&gt;Va2&gt;Va3&gt;Va4&gt;0 (4)
A multi-gradation display is produced by changing the value of the applied voltage Va.
In the prior art, the voltage V.sub.LCi across the liquid crystal capacitor C.sub.LCi is set so that when the applied voltage Va is Vai, the voltage V.sub.LCi becomes equal to the voltage V.sub.U at which the transmission of light through the liquid crystal becomes saturated. That is, ##EQU2## The partial area of the control capacitor C.sub.Ci overlapping the subpixel electrode 4.sub.i is selected so that its capacitance satisfies Eq. (5). As will be seen from Eq. (5), the reduction of the voltage to be applied to the liquid crystal capacitor V.sub.LCi of the subpixel electrode 4.sub.i Calls for a decrease in the capacitance of the corresponding control capacitor C.sub.Ci In other words, the partial area of the control capacitor electrode 2 overlapping the subpixel electrode 4.sub.i must be decreased. However, as the overlapping area decreases (the overlapping area of the control capacitor C.sub.C4 is the smallest in the above example), an error of the capacitance value C.sub.Ci increases owing to variations in the partial areas of the control capacitor electrode 2 overlapping the subpixel electrodes 4.sub.1 to 4.sub.4 which are Caused by pattern misalignments. If the liquid crystal capacitor electrode voltage V.sub.LCi greatly deviates from the transmission saturating voltage V.sub.U, an error in the multi-gradation display increases, seriously impairing the display quality.
Moreover, the adjoining subpixel electrodes 4.sub.i of each pixel must be separated by certain gaps defined therebetween, but in the prior art, since the control capacitor electrode 2 is shaped so that it does not overlap most of such gaps, no voltage can be applied to the liquid crystal layer corresponding to the gaps not overlapping the control capacitor electrode 2--this reduces the effective pixel area and hence decreases the aperture ratio of the pixel.
Besides, since the subpixel electrodes 4.sub.i are formed simply by dividing one pixel electrode in row and column directions, the center of an ON region of each pixel varies with the number of subpixels which are turned ON. Consequently, the quality of a display image is not good.